1. Field of the Invention
The present invention relates to a frictional engagement device for an automatic transmission, especially for an automatic transmission used in an automobile that engages and disengages two members to change the route of transmitting torque in an automatic transmission, so that gear ratio is changed.
2. Description of the Related Art
Generally an automatic transmission used in an automobile has one or more planetary gearsets between an input shaft which is connected to a torque converter and an output shaft which is connected to a propeller shaft or an axle shaft. Each planetary gearset has three elements, i.e. a ring gear, a sun gear and a planet-pinion carrier assembly. Gear shifts are made by changing the engagement of an element used in each planetary gearset by means of a frictional engagement device.
Suppose a shift is made by changing the engagement of one element PGIa in planetary gearset PGI which is disposed in the input side and one element PGOa in planetary gearset PGO which is disposed in the output side. Suppose more specifically, the shift is made from one gear speed (hereinafter called the first gear speed) in which element PGIa is rotating and element PGOa is not rotating, to the next gear speed (hereinafter called the second gear speed) in which element PGIa in planetary gearset PGI and element PGOa in planetary gearset PGO are both rotating while engaged with each other, and to the next but one gear speed (hereinafter called the third gear speed) in which element PGIa is not rotating and element PGOa is rotating.
The frictional engagement device disposed between the PGIa and PGOa is required to operate as follows:
(1) To make it possible to shift smoothly from the first gear speed to the second gear speed, the device gradually engages the element PGIa in planetary gearset PGI which is in a rotating condition and element PGOa which is in a stationary condition so that transfer element PGOa from a stationary condition to rotating condition;
(2) To make it possible to keep the second gear speed, the device holds the engaged condition;
(3) To make it possible to drive in engine brake condition, the device transmits a torque from the element PGOa to the element PGIa;
(4) To make it possible to shift smoothly from the second gear speed to the third gear speed, the device disengages at once when the element PGIa in planetary gearset PGI in rotating condition is stopped so that prevent the element PGOa from suffering the force of stopping; and
(5) To make it possible to have no transmitting condition, the device holds the disengaged condition.
To complete all the above listed operations, systems having two frictional engagement device and one one-way clutch device has been generally used. Because one frictional engagement device is required for the operations of (1), (2), (5) and one one-way clutch is required for (4), and another frictional engagement device is required for (3) due to the use of the one-way clutch device.
Practically, a multi-plate wet type clutch is used for frictional engagement device and a sprag type or a roller type one-way clutch is used for one-way clutch device.
FIG. 5 is a schematic view of a system having the devices described above.
In FIG. 5, casing 30 is connected to an input shaft 31. The input shaft 31 is connected to an element PGIa (not shown) in a planetary gear-set PGI (not shown) at the right (in FIG. 5) end of the input shaft(not shown). First separator plates 32 are slidably connected to the casing 30 at the inner peripheral surface thereon by means of splines or the like. The casing 30 defines a first oil pressure chamber 33 with a first piston 35.
When pressurized oil is supplied to the oil pressure chamber 33 through a oil passage 34 and the first piston 35 is displaced to the left (in FIG. 5), the first separator plates 32 are engaged with a first clutch discs 36. The first clutch discs 36 are connected to a first clutch drum 37 by means of splines or the like. The first clutch drum 37 is connected to an output shaft 39 through a one-way clutch 38. Said output shaft 39 is connected to PGOa (not shown) which is one element of planetary gear-set PGO (not shown) at the left (in FIG. 5) end of the output shaft (not shown).
Second separator plates 40 are slidably connected to the casing 30 at the inner peripheral surface thereon by means of splines or the like. The casing 30 defines a second oil pressure chamber 41 with a second piston 43. When pressurized oil is supplied to a second oil pressure chamber 41 through a oil passage 42 and a second piston 43 is displaced to the right (in FIG. 5), the second separator plates 40 are engaged with a second clutch discs 44. The second clutch discs 44 are slidably connected to a second clutch drum 45 by means of splines or the like. The second clutch drum 45 is fixedly connected to the output shaft 39.
The first piston 35 and the second piston 44 are respectively urged by springs 46 in the direction of disengagement. Check balls 47 facilitate the movement of the first piston 35 and the second piston 44 in the direction of disengagement. O-rings 48 prevent oil leaking from the first oil pressure chamber 33 and second oil pressure chamber 41.
In the above described system the required operations previously described are completed as follows:
The required operation (1) is completed by supplying pressurized oil to the first oil pressure chamber 33 through the oil passage 34. The force generated by oil pressure overcomes the force generated by the spring 46. The first piston 35 is then displaced to the left (in FIG. 5) and pushes the first separator plates 32 against the first clutch discs 36 so that they are engaged with each others. Said first clutch discs 36 are connected to the first clutch drum 37. Said one-way clutch 38, between the first clutch drum 37 and the output shaft 39, is so constructed that it transmits torque from the input shaft when the input shaft moves in a clockwise direction (as viewed from the direction of arrow A), for instance. Thus rotational torque from the input shaft 31 is transmitted to the output shaft 39, and the output shaft 39 rotates;
The required operation (2) is completed by keeping the pressure of the first oil pressure chamber 33 sufficient to overcome the force of spring 46;
The required operation (3) is completed by supplying pressurized oil to the second oil pressure chamber 41 through the oil passage 42 so that the second clutch discs 44 and second separator plates 40 are engaged. Torque from output shaft is then transmitted to input shaft, because the second clutch discs 44 are connected to the second clutch drum 45 which are fixedly connected to the output shaft 39, and the second separator plates 40 are connected to the casing 30 which is connected to the input shaft 31.
The required operation (4) is completed, automatically, by keeping the pressure of the first oil pressure chamber 33 sufficient to overcome the force of spring 46, and by setting the pressure in the second oil pressure chamber 41 to zero. Then the one-way clutch 38 frees the output shaft 39 from the input shaft 31 when the input shaft 31 is stopped; and
The required operation (5) is completed by supplying pressurized oil to neither the first oil pressure chamber 33 nor to the second oil pressure chamber 41.
By the way, a transmission is one of the heaviest and largest components in an automobile like, and the recent demand for lower emission and lower fuel consumption from automobile means that small, light transmissions are required. The above requirement is especially true in the case of automatic transmission, because automatic transmissions are generally heavier, and the amount produced is much higher.
Therefore, many kinds of small, light automatic transmissions have been proposed. For example, an automatic transmission without a one-way clutch of the sprag or roller type is disclosed by Japanese Unexamined Patent Publication No. 3-181620. Instead of a one-way clutch of the sprag or roller type, the automatic transmission has an oil outlet with a variable aperture to discharge pressurized oil which generates a force to engage an element connected to an input member with an element connected to an output member. The aperture of the oil outlet varies depending on the relative rotational position of the above two members. Oil is discharged when the input shaft is stopped, and then the above two elements are disengaged and the output member becomes free from the input member. Thus, the variable oil outlet operates as a one-way clutch of the sprag or roller type.
Therefore, the automatic transmission described above requires two clutch devices to make it possible to drive the vehicle with engine braking. Accordingly, said type automatic transmission requires space for two clutch devices, even though it can save the space required for installing a one-way clutch of the sprag or roller type.